1. Field of the Invention
This invention relates to nickel- or cobalt-chromium-molybdenum based superalloys which possess improved resistance to hydrogen embrittlement when cold-worked and aged.
2. Description of the Prior Art
Hydrogen embrittlement of steels and alloys has plagued a number of industries for many years. It has become especially a problem to oil and gas production as high concentrations of hydrogen sulfide are being frequently encountered. In search for additional production the oil and gas industry finds it necessary to drill deeper wells. Unfortunately, the current technology limits the strength levels of steels to approximately 560 MPa (80,000 psi) for sour hydrogen sulfide gas or oil wells. Above this strength level, catastrophic brittle failures in the presence of hydrogen sulfide can occur. Therefore, there is a need for substitutes for steels which will perform at high strength levels in these deleterious environments.
A major obstacle to successful drilling and completion of deep wells for oil and gas is hydrogen sulfide. When oil or gas is contaminated with hydrogen sulfide they are referred to as sour, and these sour environments cause a number of problems. A principal problem associated with these sour environments is their tendency to cause catastrophic, brittle failure of the metals used in drilling and completing the wells. This brittle failure can be a problem particularly when using the high strength steels needed for producing deep high pressure sour wells.
Catastrophic, brittle failure has been experienced with tubing, casing drill pipes, wire lines and related equipment. Sudden failure of such equipment can occur with little visible corrosion and no detectable plastic deformation. Moreover, brittle failures can occur in metals subjected to apparently safe loading and often after extended periods of satisfactory service. Such failures can result in loss or blowout of the well with disastrous consequences in terms of life and money.
Although the mechanism of such brittle failure is not fully understood, it is believed to be caused by entry of atomic hydrogen (H) into the metal. The hydrogen drastically reduces the ductility and causes the metal to crack. This phenomenon is commonly called hydrogen embrittlement. The mechanism of hydrogen embrittlement is more complex in the presence of hydrogen sulfide because of additional chemical reactions. In the presence of hydrogen sulfide, this mechanism is commonly called hydrogen sulfide embrittlement or sulfide stress cracking. The term hydrogen embrittlement, however, will be used herein in reference to brittle failure regardless of the hydrogen source.
Hydrogen sulfide is one source of the hydrogen and a common one. It may originate from water containing sulfur compounds, inflow of formation fluids, bacterial action on sulfates in drilling fluids, thermal degradation of drilling fluid additives and chemical reactions between sulfur compounds and metal. Hydrogen itself rather than hydrogen sulfide may be the cause of the brittle failure. Uncombined hydrogen can be generated from a number of sources including corrosion processes in the drilling fluids, bacterial action, and thermal degradation of organic additives.
Further discussion of hydrogen embrittlement problems in oil field production is described in the following references.
Greer, J. B., "Metal Thickens and Temperature Effects in Casing and Tubing Design for Deep, Sour Wells", paper SPE 3968, presented at SPE-AIME 47th Annual Fall Meeting, San Antonio, Tex., Oct. 11, 1972.
Watkins, M. and Greer, J. B., "Corrosion Testing of Highly Alloyed Materials for Deep Sour Gas Well Environments", paper SPE 5622, presented at SPE-AIME 50th Annual Fall Meeting, Dallas, Tex., Sept. 28-Oct. 1, 1975.
Greer, J. B., "Factors Affecting the Sulfide Stress Cracking Performance of High Strength Steels", Materials Performance, Vol. 14, pp. 11-22, January, 1975.
Kane, R. D. and Greer, J. B., "Sulfide Stress Cracking of High-Strength Steels in Laboratory and Oilfield Environments", paper SPE 6144, presented at SPE-AIME 51st Annual Fall Meeting, New Orleans, La., Oct. 3-6, 1976.
Kane, R. D., Watkins, M., Jacobs, D. F. and Hancock, G. L., "Factors Influencing the Embrittlement of Cold Worked High Alloy Materials in H.sub.2 S Environments", Corrosion, 33 (9) 309-320 (September, 1977).
Several suggestions have been proposed to minimize equipment failure caused by hydrogen embrittlement. For example, inhibitor additives, protective coatings and metallurgical compositions have been proposed. Among the more promising metallurgical compositions proposed for use in deep sour wells are the highly alloyed metals such as high-strength super austenitic stainless alloys composed principally of nickel (and/or cobalt), chromium and molybdenum. One example of such superalloys is Hastelloy C-276* alloy (manufactured by Cabot Corporation) which is a nickel-based corrosion-resistant alloy with additions of 16% chromium, 16% molybdenum, 4% tungsten, 5% iron, and with the carbon and silicon contents maintained as low as practically possible. This alloy has been extensively described, e.g., British Pat. No. 1,160,835 and Canadian Pat. No. 859,062; R. B. Leonard, Corrosion NACE, 25 (5) 222 (1969); R. B. Leonard, Chemical Engineering Progress, 65 (7) 84 (1969); R. W. Kirchner and W. L. Silence, Materials Protection and Performance, 10 (1) 11, (1971); and R. B. Leonard, Metal Progress, pp. 87-88, March, 1971. FNT *A registered trademark of Cabot Corporation.
Another example of such a super alloy is Hastelloy C-4* alloy (also manufactured by Cabot Corporation) which is a nickel-based corrosion-resistant alloy with additions of 16% chromium, 15% molybdenum, less than 2% iron, a maximum of 0.5% tungsten, less than 0.5% aluminum, less than 1% cobalt, less than 0.5% manganese, up to 0.5% titanium, less than 0.01% carbon and less than 0.03% silicon. This alloy has been described, e.g. British Pat. No. 1,454,814; M. A. Streicher, Corrosion NACE, 32 (3) 79 (1976); F. G. Hodge and R. W. Kirchner, Corrosion NACE, 32 (8) 332 (1976); T. S. Lee and F. G. Hodge, Materials Performance, pp. 29-36 (September 1976) and B. E. Paige, Materials Performance, pp. 22-28 (December, 1976). FNT *A registered trademark of Cabot Corporation.
As reported by M. Watkins and J. B. Greer, J. Pet. Tech., 28 698 (1976) these superalloys not only have good corrosion resistance but appear to resist embrittlement in hydrogen producing environments, even at high strength levels under the limited test conditions (i.e., unaged and uncoupled to steel).
Other proposed methods to reduce embrittlement problems in conventional steel components include contacting the metal equipment with hydrogen sulfide at temperatures above 150.degree. F., avoiding use of pipe that has been cold-straightened or cold-worked, using biocides to control sulfate reducing organisms, maintaining a high pH (9-10.5) within the well and using thicker wall pipe to reduce high stresses. Although the oil and steel industries have made significant efforts to resolve the hydrogen embrittlement problems of steels, most of the proposed methods are essentially ineffective to totally protect metal equipment from brittle failure.
It was recently discovered that four cold-worked nickel and cobalt-base superalloys (Hastelloy C-276*, Hastelloy C-4*, Inconel 625** and MP35N***) can be susceptible to hydrogen embrittlement in corrosive aqueous environments when coupled to steel and when stressed in the transverse direction and that the embrittlement susceptibility is substantially affected by heat treatment (Kane, R. D., Watkins, M., Jacobs, D. F. and Hancock, G. L., Corrosion, 33 (9), 309-320 (September, 1977)). Their susceptibility to hydrogen embrittlement is increased by aging treatments conducted within the temperature range of 200.degree. C. and 500.degree. C. It was recently discovered, however, that in the case of cold-worked multi-phase type alloys, such as in the case of MP35N***, which are heat-treated at a temperature ranging from about 1300.degree. F. (704.4.degree. C.) to about 1600.degree. F. (871.1.degree. C.) for a time interval within the range from about 1/4 to 100 hours, a substantial resistance to hydrogen embrittlement is obtained. A more complete description of this innovation is described in U.S. Ser. No. 767,609, filed Feb. 10, 1977, abandoned entitled "Alloys Having Improved Resistance to Hydrogen Embrittlement", the disclosure of which is incorporated herein by reference. FNT *A registered trademark of Cabot Corporation. FNT **A registered trademark of Huntington Alloys, Inc. FNT ***A registered trademark of Standard Pressed Steel Co.
R. M. Latanision and H. Opperhauser, Jr., Met. Trans., 5, 483 (1974) recently indicated that tramp impurity segregation in materials correlates to increased susceptibility to hydrogen embrittlement.
As oil and gas wells are drilled deeper and as higher concentrations of hydrogen sulfide are encountered at higher pressures, there is a substantially unfilled need for high-strength superalloy material which has improved resistance to hydrogen embrittlement.